AB0108 INFLAMMATION IN axSpA AS A DISRUPTOR OF BONE METABOLISM – THE EFFECT OF PATIENTS’ SERA ON AN IN VITRO BONE MODEL
Authors: E. Sebova, P. Skubica, M. Husakova, P. Dankova
Journal: Annals of the Rheumatic Diseases (2022)

Background In axial spondyloarthritis (axSpA), two opposing processes of bone resorption and neogenesis are closely related, with a common denominator – chronic inflammation. This pathological condition affects osteoblasts, among other cells, residing in sites of inflamed tissue, but it can also activate mononuclear precursors in the blood to form osteoclasts [1,2]. To this day, it is not yet clear how the interplay between osteoblasts and osteoclasts affects pathogenesis of axSpA. Objectives The aim was to evaluate the effect of sera from patients with different forms of the disease, non-radiographic (nr-axSpA) and radiographic (r-axSpA) including ankylosing spondylitis, in comparison to the effect of sera from healthy individuals (axC) on osteoblasts and osteoclasts co-culture. Methods The inflammation characteristic for each diagnose was simulated by age- and sex-matched pools of 12 nr-axSpA and 12 r-axSpA patients‘ sera, healthy condition by a pool of 12 axC sera. We used THP-1 monocytic cell line and SaOS-2 osteosarcoma cell line co-culture to model bone physiology. After THP-1 adhesion, SaOS-2 cells were added with 10 % of nr-axSpA, r-axSpA or axC sera, respectively, in the media for 14 days. Activity of osteoblastic alkaline phosphatase (ALP) and osteoclastic enzymes tartrate-resistant acid phosphatase (TRAP), cathepsin K (CTSK) and carbonic anhydrase II (CAII) on days (D) 1, 7 and 14 were measured. Simultaneously, we examined metabolic activity (MTS assay), cell proliferation (PicoGreen assay) and mRNA expression of selected genes in co-culture. Results In all three studied co-cultures the ALP activity increased with length of the cultivation; nevertheless, the onset of the activity in D1 was more rapid in axC sera compared to nr-axSpA (P&lt;0.05) and r-axSpA (P&lt;0.001). The ALP activity increase from D1 to D14 was 4.9 fold in axC, 5.7 fold in nr-axSpA and 6 fold in r-axSpA. Moreover, rise of ALP gene expression corresponds well with rise of ALP activity. TRAP activity peaked in all sera on D7 and decreased again towards D14, mirroring the same mRNA expression profile. The TRAP activity on D7 was highest in axC sera and lowest in r-axSpA (P&lt;0.01). Gradual drop of CA II activity from D1 to D14 was detected in co-cultures with axC and nr-axSpA sera; in r-axSpA, however, the activity increased 1.42 times from D7 to D14 and was thus significantly higher on D 14 compared to cultures in other two sera (P&lt;0.0001). In all cultures, CTSK activity increased from D7 to D14. Activity was highest in culture with axC sera (nr-axSpA D7 and D14 P&lt;0.0001; r-axSpA D7 P&lt;0.0001, D14 P&lt;0.05). A gradual increase in cell proliferation was observed in axC sera and was thus significantly higher compared to both patients’ sera on D14 (P&lt;0.0001). In contrast, the proliferation in patients‘ sera peaked on D7 with the significantly highest D7 proliferation observed in r-axSpA (P&lt;0.05). While the metabolic activity per cell followed U shape trend in all co-cultures, the changes were more distinct in patients‘ sera: 2 fold drop in r-axSpA, 1.6 fold in nr-AxSpA and 1.3 fold in axC sera on D7 followed by increase of 3.4 fold in r-AxSpA, 2.9 fold in nr-AxSpA and 1.8 fold in axC on D14. Conclusion Data acquired from the in vitro bone model point to an imbalance in the enzymatic activity of osteoclasts and to changes in the metabolic activity and proliferation of both, osteoblasts and osteoclasts. These changes seem to be caused by inflammatory agents in patients’ sera. However, the changes are not the same in both forms of disease, which may explain differences between nr- and r-axSpA symptoms. References [1]Sieper, J., Braun, J., Dougados, M., &amp; Baeten, D. (2015). Axial spondyloarthritis. Nature reviews. Disease primers, 1, 15013. https://doi.org/10.1038/nrdp.2015.13 [2]Van Mechelen, M., Gulino, G. R., de Vlam, K., &amp; Lories, R. (2018). Bone Disease in Axial Spondyloarthritis. Calcified tissue international, 102(5), 547–558. https://doi.org/10.1007/s00223-017-0356-2 Acknowledgements Grant support: Progres Q43 and SVV260436 Disclosure of Interests None declared